A water-absorbing resin (SAP/Super Absorbent Polymer) is a water-swelling and water-insoluble polymer gelling agent, and has been frequently used principally as disposable applications, as absorbing articles such as disposable diapers, sanitary napkins, a water retention agent for gardening and agriculture, an industrial use water stop material or the like. As such a water-absorbing resin, those using many monomers or hydrophilic polymers as raw materials have been proposed. Among them, in particular, a polyacrylic acid (or salt thereof)-based water-absorbing resin obtained by using acrylic acid and/or a salt thereof as a monomer has been used industrially in the highest amount, in view of its high water-absorbing performance.
Such a water-absorbing resin is produced through a polymerization step, a drying step, a pulverization step, a classification step, a surface cross-linking step or the like (Patent Documents 1 to 3). Accompanying with higher performance of disposable diapers as a principal application, a water-absorbing resin has also been required many functions. Specifically, many properties including, as well as high water absorption capacity, gel strength, water-soluble content (Patent Document 4), water absorbing speed, absorption against pressure (Patent Document 5), liquid permeability, particle size distribution, urine resistance, antibiotic property, impact resistance, powder fluidity, deodorant property, coloring resistance, low powder dust and the like have been required to be imparted to the water-absorbing resin. Therefore, many proposals such as surface cross-linking technology, additives, changes of production steps and the like have been offered also in other than the above or following Patent Documents 1 to 23.
In recent years, with increase of use amount (for example, equal to or higher than 50% by weight) of the water-absorbing resin in disposable diapers, liquid permeability has been considered as a more important factor. And, many improvement methods or modification technologies for liquid permeability under load or liquid permeability without load, such as SFC (saline flow conductivity/Patent Document 6) or GBP (Gel Bed Permeability/Patent Documents 7 to 9) have been proposed.
In addition, in such properties, many combinations of a plurality of parameters including liquid permeability have also been proposed, and there have been known a technology for specifying frangibility index (FI) (Patent Document 10), technology for specifying water absorbing speed (FSR/Voretex) or the like (Patent Document 11), and a technology for specifying product of Saline Flow Conductivity (SFC) and wicking absorption after 60 minutes (DA60) multiplied (Patent Document 12).
Further, as enhancement methods for liquid permeability such as SFC or GBP, there have been known a technology for adding gypsum before or during polymerization (Patent Document 13), a technology for adding a spacer (Patent Document 14), a technology for using a nitrogen-containing polymer having 5 to 17 moles/kg of a nitrogen atom which is capable of protonation (Patent Document 15), a technology for using polyamine and a polyvalent metal ion or a polyvalent anion (Patent Document 16), a technology for coating a water-absorbing resin having a pH of below 6 with polyamine (Patent Document 17), and a technology for using polyammonium carbonate (Patent Document 18). Other than these, a technology for using polyamine with a soluble content of equal to or higher than 3%, and a technology for specifying wicking index (WI) or gel integrity (Patent Documents 19 to 21) have been known. In addition, for improving coloring and liquid permeability, a technology for using a polyvalent metal salt while controlling methoxyphenol which is a polymerization inhibitor in polymerization (Patent Documents 22, 23) has also been known.